Saturday, June 20, 2009

Eati Carbs does not delay the onset of fatigue during exercise.

This one is interesting given the normal advice and the practice of many athletes. Even recreational joggers seem unable to go more that 10 minutes without slurping from a powerade or lucazade bottle. Such a practice might not have any effect. From a paleo perspective, it would make sense. Why would you be built to need regular refueling while on a run, on a hunt?

Carbohydrate ingestion during exercise does not delay the onset of fatigue during submaximal cycle exercise.

The objective of this study was to evaluate the effect of the ingestion of carbohydrate (CHO, in the form of maltodextrin) or placebo (PLAC, in the form of gelatin) on the physical performance of cyclists during submaximal exercise until fatigue on an ergometric cycle. Nine volunteers exercised on 2 separate occasions at least 2 days apart. On each occasion, after 48 hours of a balanced diet, they pedaled at approximately 66% &OV0312;o2peak until fatigue. Every 15 minutes, 150 mL of water and 18 capsules, containing either 0.5 g of CHO or PLAC (~0.13 of body weight), were ingested in accordance with a double-blind, randomized protocol. The results show that after 40% of total exercise time, blood glucose levels in the CHO test returned to baseline levels. However, in the PLAC trial these levels failed to return to baseline levels, remaining lower than levels recorded in the CHO test after 60% of total exercise time. Despite these results, CHO ingestion failed to delay the onset of fatigue (CHO: 91.8 +/- 10.1 minutes vs. PLAC: 93.3 +/- 16.1 minutes; p = 0.87). In practical terms, coaches and trainers should consider that CHO ingestion in previously fed users does not delay the onset of fatigue during submaximal cycle exercise.


Jeff said...

Amazing how many I see at the gym eat before working out(usually a free banana) and after. The idea of "fueling up" for a workout is so silly if the person is overfat. Carrying around an extra 20# the person could go for hours without needing to eat more. Just silly.

Good post, thanks.

Drs. Cynthia and David said...

I agree that you shouldn't need any supplementation during short exercise like this (at least not for a few hours). But when David was a high carb eater, he couldn't go very long without food- he's quite the opposite now.

Also, this study seems flawed in that they used gelatin as their placebo. The composition of gelatin on shows that gelatin is relatively rich in branched chain amino acids, which are used during endurance exercise. It is possible that the gelatin acted as a fuel substitute, or to generate more glucose, or delayed the trp induced fatigue that comes with prolonged exercise (trp is a serotonin precursor, and enhanced transport of trp across the blood brain barrier by amino acid transporters - when the blood concentration of branched chain amino acids like val and leu is reduced - results in fatigue). If anything, this study makes me think maybe gelatin could be decent fuel to ingest while exercising.

See this paper too:

L. Wu said...

I've read that you don't really need extra carbs in general when you play sports or exercise for less than ninety minutes.

On the other hand, if you're doing an Ironman, or playing a club sport that sometimes has you play five one-hour games in a row (as I do), you probably could benefit from a small amount of carbs and proteins along the way.

For example, see this Chinese University HK study which has participants consume Carbohydrate-electrolytes after a 90-minute run, rest for a few hours, and run again:

It's important to note that the study you cite studies *submaximal* cycle effort, this other study looks at carbohydrate-electrolyte ingestion during intermittent high-intensity running:

dr. m.c. said...

yup yup yup agree with the above posts, surely it's no surprise that if you've eaten, have sufficient carb stores for the muscles to access, and aren't draining them with your level of "submax" effort, you don't need to top up fuel - you have enough on board.

also agree in particular that using gel. is not likely a good placebo.

i kinda doubt this study would have made it in journal of nutrition. but i still want to read the whole thing :)